You should do fine with the elliptical
I have done some research on the effectiveness of treadmills verses elliptical trainers and they supposedly give very similar results working for the same amount of time at the same intensity. The difference on the elliptical is the lack of impact on the lower body. If you have an injured knee I assume this is the reason you are looking at them. The lower impact actually gives the feeling of lower perceived exertion so you don't "fee"l like you are working as hard, which is why many people like them. I personally like both treadmills and ellipticals and have found both to be equally effective for me. I think you will do well with the elliptical trainer. Good luck!